Method of cleaning asphalt tanks



2,715,594 Patented Aug. 16, 1955 METHOD OF CLEANING ASPHALT TANKSCharles W. Garrison, Fort Mitchell, Ky., assignor to The Standard OilCompany, Cleveland, Ohio, a corporation of Ohio No Drawing. ApplicationAugust 26, 1952, Serial No. 306,493

1 Claim. (Cl. 134-22) This invention relates to a process of cleaningasphalt tanks.

Before referring in more detail to the method which comprises myinvention, it is desirable to describe briefly the method by whichasphalt is made and stored and the equipment used, which gives rise tothe problem which is solved by my invention.

In processing crude petroleum oil the lighter fractions which can bedistilled at atmospheric pressure such as gasoline, kerosene, etc. aredistilled off and the product remaining is known as reduced crude. Thisis further distilied under a vacuum in order to reduce the temperaturebelow cracking temperatures. For this purpose a large fractionatingcolumn is often employed which is referred to as a pipe still. A numberof distilled fractions are obtained such as a gas oil fraction,fractions comprising various viscosity lubricating oil stocks andcylinder stock, and fractions containing wax such as parafi'indistillate and wax slops. The residue which remains after all theoverhead fractions have been removed is referred to as pipe stillbottoms. The exact composition of the pipe still bottoms will dependsomewhat on the source of the crude and the extent to which variousfractions are distilled overhead. It may be modified by blending backsome of the distilled fractions if desired. Pipe still bottoms, amongothers, are utilized in the preparation of asphalt. Other materialswhich can be used include residue from thermal cracking referred to ascracking coil tar. Any of these heavy black viscous residual materialsis referred to as petroleum residuum.

In converting a petroleum residuum to asphalt, one method is to place itin a large tank and blow air and sometimes steam through it to oxidixeand reduce the residuum. The extent of the oxidation of any particularstock due to the air blowing largely determines the hardmass or softnessof the asphalt, i. e., whether it has a high or low penetration.

The oxidation is generally carried out in a large tank such as from 10to 100 feet in diameter and about 15 to 75 feet high. Spaced a shortdistance from the bottom, such as 2 inches to 3 feet, are a series ofpipes generally arranged to cover the bottom area such as thatresembling the spokes of a Wheel. The pipes have holes in the bottomportions. The tank is filled or partly filled with the petroleumresiduum to be converted to asphalt and compressed air is introducedinto the pipe system and blown out through the holes. The holes, beingin the bottom of the pipes, direct the air towards the bottom of thetank and the air bubbles up through the asphalt which is maintained in aliquid condition by heating it to a temperature in the range of 360 to550 F. After this operation has been continued for the required lengthof time, the air is turned off and the asphalt is withdrawn from thetank at a sufiiciently high temperature so that it remains fluid. In allof these operations it is desirable to keep the temperature sufiicientlyhigh so that the material remains liquid.

Notwithstanding the use of these temperatures, a layer of hard rock-likeasphaltic material and coke accumulates all over the interior, includingthe bottom, of the tank. This is to be distinguished from ordinaryasphalt because this layer does not liquify and flow from the tank withthe asphalt.

A similar hard layer of material forms during use of tanks for storing,mixing and transporting asphalt. Many of such tanks include pipesthrough which air may be blown for the purpose of agitating and mixingthe asphalt,

or such pipes may be readily installed.

Periodically it becomes necessary to inspect or repair these asphalttanks and the practice generally employed heretofore is to drain all theasphalt that will flow from them at an elevated temperature, cool themto ambient temperature and then to chip out the rock-like layer of hardasphaltic material which may be anywhere from an inch or so to four feetin thickness and firmly adherent to the bottom and sides of the tank.Often the accumulations of coke and rock-like asphaltic materialresemble a series of small mounds inside the tank. Apparently agitationresulting from air blowing would prevent the formation of a smoothlayer. The valleys be tween the various mounds would be filled withasphalt which would not drain out naturally. Accordingly, the chippingout often involved the removal of asphalt, rocklike layers of hardasphaltic material, and coke. This is a dirty, unpleasant,time-consuming, and expensive job. Tank cleaning in an asphalt plant isa major and costly problem which has not been solved heretofore in thefield.

I have discovered in accordance with my invention that this hardrock-like layer of asphaltic material and coke, which cannot normally bedrained, may be removed by introducing an amount of kerosene or otherlike hydrocarbon having a boiling point in the range of 350 to 600 F. toprovide a depth sufiicient to cover the material in the tank to becleaned. Live steam is then forced through the pipes and blown outthrough the holes towards the bottom of the tank. The steam is blownagainst the surface of the tank from which the deposit is to be removed.The steam apparently functions to assist in loosening asphalt and thehard asphaltic material and coke and facilitate the dissolving of atleast a portion of them in the kerosene. In addition, the water from thecondensed steam inside the tank tends to float some of the asphalt andloosened material and prevent it from resettling. In this way at least aportion of the asphalt that is loosened may be suspended in theinterface between the kerosene and the water, and any of the remainingundissolved asphalt is prevented from adhering to the bottom of the tankby the water, which being heavier than the hydrocarbon, forms the bottomlayer of liquid.

The blowing of steam as described may be continued until the rock-likelayer is removed, anywhere from a few hours to a week. I have found thatabout two to three days is generally adequate for the treatment. At theend of this time the steam is shut oh and the kerosene and water isdrained from the tank. Surprisingly, the tank was very clean except fora very slight film of sludge which was soft and could be readily scrapedoff or washed off without any difliculty. The solvent can be re-used orused in making cut-back asphalt.

As illustrative of a practical embodiment of my invention, an asphalttank 80 feet in diameter and 42 feet high equipped with the air blowingpipes was attempted to be cleaned at two different times ten years apartby the previously known methods. In these attempts various solvents havebeen employed by themselves and steam has been employed by itselfwithout removing the de posit. These attempts required ten mens time fora r 3 a period of four weeks and the tank'was only partiall cleaned inthat it was not possible to remove the accumulated hard material down tothe bare metal.

In cleaning this same tank by the method of the invention, some tenyears after the last previous attempt to clean it, all of the hotasphalt which would flow from the tank was pumped out and immediaelythere was pumped into the tank a solvent of the kerosene type normallyused for making #3 cut-back asphalt. The amount of solvent wassufiicient to form a layer two feet deep in the tank. Live steam wasthen introduced through the air holes for a period of three days. Thishad the eflect of heating, dissolving, and cutting the ac-V eumulateddeposit while at the same time the condensed water from the steam 'wasgetting under the asphalt on the floor of the tank and lifting it upwhere it could be dissolved in' the solvent. At the end of this time thetank was emptied and material pumped out largely in an emulsified state.The man-holes and clean-up plates were taken from the tank'and it wasfound that praci tically all of the deposit had been removed. Whatlittle 'i'emained was easily washed out in an emulsified state withwater from a fire hose. The Water was drained from the tank and the baremetal plate was exposed, which could readily be inspected and repaired.

It will be obvious that other minor variations may be "asphalt.

made in the details of the process depending upon the type of equipmentthat is employed in making the All such variations are intended to beincluded within my invention as set forth by the claim.

I claim:

A method of cleaning a tank used for the conversion of petroleumresiduum to asphalt to remove a hard solidified layer of material whichhas accumulated as a result of said conversion on the interior of thetank and which is solid at asphalt liquifying temperatures, whichcomprises introducing into the tank a layer of a hydrocarbon fractionboiling generally within the range of 350 to 600 F. and blowing livesteam below the hydrocarbon layer toward the solidified layer ofmaterial on the tank, continuing the steam blowing for 'a period of froma few hours to a week until the material has been separated from thetank surface, and withdrawing from the tank the hydrocarbon and thewater from the condensed steam together with the separated material. 7

References Cited in the file of this patent UNITED STATES PATENTS

